1. Field of the Invention
The present invention relates to an improved method for efficiently reducing NO.sub.x content in the waste gas which contains a large amount of impurities such as dust generated in a glass-melting furnace.
2. Prior Art of the Invention
Glass-melting furnace among the industrial furnaces generates waste gas containing a relatively high concentration of NO.sub.x since it is heated to about 1500.degree. C. during operation. This waste gas is extremely dirty, containing a mixture of glass material, dust and substances evaporated from the surface of molten glass. In the case of boilers, measures for reducing NO.sub.x content have been developed such as use of low-NO.sub.x burners, a two-step combustion system as well as a catalytic method for reduction of NO.sub.x in waste gas with NH.sub.3 or CO, so-called waste gas denitration method.
However, in glass-melting furnaces, such modification of combustion tends to lower the glass-melting temperature and to disturb the temperature distribution in the furnace required for obtaining a good quality of glass product, resulting in defects in the glass product such as bubbles or streaks. Meanwhile, the use of a catalyst for waste gas denitration is found impractical, because the catalyst is liable to be poisoned or clogged with dust in the exhaust gas. Thus all the methods are inapplicable to glass-melting furnaces.
For these reasons, a non-catalytic denitration method is deemed promising as a means for reducing NO.sub.x content in glass-melting furnaces. A non-catalytic method for reducing NO.sub.x content using NH.sub.3 is known, but the effective temperature range in this method is 800.degree.-1000.degree. C. Therefore it would be virtually impossible to reduce NO.sub.x in a practical glass-melting furnace unless a special device is additionally installed.
In Japanese Patent Laid-open specification No. 8361/1978, another method is proposed in which hydrocarbons, oxygen-containing derivatives thereof, mixtures thereof, or organic mixtures of other materials (liquefied petroleum gas, kerosene, city gas and the like) with the hydrocarbons and/or oxygen-containing derivatives thereof (hereinafter referred to as hydrocarbons or the like) are added to hot waste gas and brought into contact with said gas. According to this method, hydrocarbon or the like is introduced into the furnace through the vicinity of a port, regenerator, heat exchange chamber or flue. However, three serious problems are encountered in application of this method to practical glass-melting furnaces.
One of them is as follows: Hydrocarbon or the like introduced into the furnace through the regenerator is burned with the residual oxygen in the waste gas. In a practical furnace the temperature in the vicinity of the regenerator exceeds at least 1000.degree. C., but the concentration of the residual oxygen in the waste gas from the combustion chamber is normally less than about 3%, which is far lower than the oxygen concentration of the atmosphere in which ordinary combustion takes place. Thus the introduced hydrocarbon or the like burns very slowly, producing long flame, which comes into contact with the sidewall bricks and the checker bricks of the regenerator. A similar phenomenon will take place even when the introduction is made through the vicinity of the port, because of the short distance between the port and the regenerator.
The above phenomenon causes the following problem: The regenerator of glass-melting furnaces has a relatively short life as it is used under a severe condition. The refractory material is generally vulnerable to the reducing atmosphere, especially when it comes into contact with hot flame, because the transition elements contained therein (including not only those of the refractory constituents but also contained as impurities) change to lower valency state, thereby causing easy deformation with a decreased resistance to fire. In the conventional regenerator, chromia-magnesia bricks are mainly used as a material for checker bricks, so that the above-mentioned risk of embrittlement is all the more serious. To avoid such a risk, the flame caused by the supplied hydrocarbon or the like must be prevented from contacting the refractory wall of the regenerator.
The second problem is as follows: To attain an effective reduction of NO.sub.x content, the supplied hydrocarbon or the like must be brought into sufficient contact with the waste gas; however, since the regenerator has usually a large sectional area (i.e. large volume) and the supply amount of hydrocarbon or the like per one burner is considerably small compared with the amount of the main fuel consumed in the combustion chamber, the desired effect cannot be attained unless a large number of additional entrances are provided for introducing hydrocarbon or the like. Attaching such additional entrances in a furnace at work is almost impossible, excepting the case of installing a new furnace.
The third problem is as follows: A major part of the heat energy of the supplied hydrocarbon or the like is consumed in heating the checker bricks and partially in heating the secondary air so that utilization of the energy is not efficient.